1. Field of the Invention
The present invention relates to a positive temperature coefficient (hereinafter referred to as PTC) thermistor device which includes a PTC element and more particularly, it relates to support structures for a PTC thermistor element.
2. Discussion of the Background
PTC thermistor devices are used with motor drive circuits or the like in electric refrigerators, for example. A PTC thermistor element which is employed in a PTC thermistor device is one of semiconductor temperature sensor devices, which noticeably increases its resistivity in a non-linear or exponential manner as the temperature increases and which has a positive temperature coefficient as a whole. Typically, such PTC thermistor element is held or housed in its associative vessel such as an enclosure or casing, and is attached to a motor drive circuit, for instance.
PTC thermistor elements are those devices having a function of suppressing flow of current by heat generation. However, where the PTC thermistor element is abnormal in operation, thermorunaway can arise due to flow of overcurrent, causing the element to rapidly increase in temperature, which would lead to element destruction.
Conventionally, even upon occurrence of the element destruction mentioned above, it is not possible to reliably interrupt or cut off the flow of overcurrent, which would result in an increase of the risk of combustion of the casing or the like due to continuous flow of such overcurrent. In view of this, it has been long desired that once the aforesaid element destruction occurs, any possible flow of overcurrent be successfully interrupted or shut down eliminating accidental firing or equivalents thereto, thus increasing reliability.
A conventional PTC thermistor device is shown in FIG. 15. In this figure, reference numeral 1 indicates a PTC thermistor element, 2 indicates electrodes of the PTC thermistor element 1, 3 indicates a support member of the PTC thermistor element 1, 5 indicates a casing, 6 and 7 indicate terminal sections, 8 and 9 indicate spring members as integral with the terminal sections 8 and 9 respectively, 10 indicates a spring contact piece (contact section to be contacted with electrode 2), and 11 indicates a spring contact piece support section.
The prior art illustrated in FIG. 15 is arranged such that the plate-shaped PTC thermistor element 1 having electrodes 2 formed on its both principal surfaces is housed within the insulating casing 5 and is elastically held by spring members 8 and 9 each consisting of an elastic or resilient metal plate, while causing the spring members 8 and 9 to be secured to the terminal sections 6 and 7. Each spring member 8 and 9 includes a spring contact piece support section 11 of a substantially constant width extending in parallel to the electrodes 2 on the principal surface of the PTC thermistor element 1, and spring contact pieces 10 (contact sections) which extend from respective ends of this spring contact piece support section 11 and being bent toward the principal surface electrodes 2 of PTC thermistor element 1 to become into contact with the electrodes 2 and thereafter being bent toward the spring contact piece support section 11 (see Unexamined Japanese Utility-Model Publication No. 3-99402).
Another PTC thermistor device belonging to the prior art is shown in FIG. 16. In this figure, numeral 15 indicates a PTC thermistor element, 16 indicates electrodes of the PTC thermistor element 15, and 17 and 18 indicate terminals. The prior art illustrated in FIG. 16 is arranged such that the PTC thermistor element 15 having electrodes 16 formed on two outer opposite surfaces thereof is elastically supported by a pair of terminals 17 and 18 with elasticity. In this case, the PTC thermistor element 15 is held between both terminals 17 and 18 (PTC thermistor element 15 is supported at three points) while the contact sections 19, 20 and 21 of the terminals 17 and 18 are asymmetrical on both surfaces of PTC thermistor element 15 (see Unexamined Japanese Utility-Model Publication No. 3-99402).
Yet another PTC thermistor device belonging to the prior art is shown in FIG. 17, and FIG. 18 is a cross-sectional view taken along lines 18--18 of FIG. 17. In these figures, numeral indicates a PTC thermistor element, 26 indicates electrodes of PTC thermistor element 25, 28 indicates a casing, 29 and 30 indicate spring members, 31 and 32 indicate terminals integral with spring members 29 and 30 respectively.
The mounting/assembly process of the PTC thermistor element into the PTC thermistor device illustrated in FIGS. 17 and 18 is shown in FIGS. 19 and 20, and FIG. 21 is a flow chart wherein S1 to S6 designate the respective steps of this process. In FIGS. 19 and 20, 34 indicates a guide film.
An explanation will now be given of the PTC thermistor element mounting/assembly process of the prior art shown in FIGS. 17 and 18.
After the terminals 31 and 32 with spring members 29 and 30 are attached to the casing 28, the PTC thermistor element 25 is then inserted between the spring members 29 and 30, allowing PTC thermistor element 25 to be elastically supported by the spring members 29 and 30.
Incidentally, since the electrodes 26 (e.g. silver electrodes) are provided on both sides of the PTC thermistor element 25, when the PTC thermistor element 25 is simply inserted directly between the spring members 29 and 30, the electrodes 26 could come into contact with the spring members 29 and 30 during insertion, which would result in rubbing off and scars. To avoid this, the PTC thermistor element 25 is inserted into the casing 28 by the following assembly process while referring to FIGS. 19 to 21.
First of all, pre-manufactured components are prepared including the casing 28, the terminals 31 and 32 with the spring members 29 and 30, and the PTC thermistor element 25 (step S1 in FIG. 21); then, assembling thereof is started. Terminals 31 and 32 are mounted within the casing 28 (step S2 in FIG. 21); thereafter, two guide films 34 are loaded into the casing 28 (step S3 in FIG. 21). In this case, the two guide films 34 are inserted and set between the spring members 29 and 30.
Next, the PTC thermistor element 25 is inserted between the two guide films 34 in a way shown in FIG. 19 (see also step S4 in FIG. 21). In other words, the PTC thermistor element 25 is pushed thereinto from its upper side. Thereafter, as shown in FIG. 20, while causing the PTC thermistor element 25 to be kept compressed in a direction designated by the arrow shown (downward), the guide films 34 are pulled out in directions indicated by the arrows shown therein (upward) for release to the outside (step S5 in FIG. 21). In this way, the spring members 29 and 30 are in contact with the electrodes 26 of the PTC thermistor element 25, completing the assembly process of PTC thermistor element 25 (step S6 in FIG. 21).
However, the prescribed prior art devices described above encounter the folowing problems.
Where the PTC thermistor element is abnormal in operation, thermorunaway can arise due to flow of overcurrent, causing the element to rapidly increase in temperature, which would lead to element destruction. In such case, when resultant fragments of the PTC thermistor element have dropped down onto the lower part of the casing, electrical circuity will be interrupted. However, the fragments can sometimes be trapped between terminals and under this condition, even where the electrical circuitry per se is shut off, some fragments staying between terminals can behave badly to inhibit intended electrical interruption of the circuitry. If this is the case, the overcurrent might continue flowing, thereby raising the temperature abnormally, which could in the end result in combustion of the casing or the like.
Especially, the PTC thermistor device belonging to the prior art as shown in FIG. 15 is designed such that the PTC thermistor element is supported by multiple contact sections provided at the terminals. Accordingly, after the PTC thermistor element is cracked, its fragments hardly fall down onto the lower part of the casing. This design could sometimes cause burning of the casing or the like as stated above.
With the PTC thermistor device belonging to the prior art as shown in FIG. 16, the PTC thermistor element is easily destructible due to the three-point support of the PTC thermistor element. However, in view of the fact that all the parts supporting the PTC thermistor element are conductive terminals, it is rather difficult upon occurrence of element destruction to interrupt the overcurrent flow unless the destroyed element fragments perfectly fall down onto the lower part of the casing. In other words, if a few fragments are left on the lower part of the casing, the possibility that the flow of overcurrent through the electrodes of such destructed PTC thermistor element and/or terminals continues remains high, which would sometimes result in firing accidents as discussed previously.
Another problem encountered in the PTC thermistor devices of the prior art is where the PTC thermistor element is mounted for assembly into the casing wherein guide films are employed, as shown in FIGS. 19 and 20 for example. Such PTC thermistor element assembly process suffers from the following problems.
(a) Loading and unloading of the guide films require time consuming and troublesome works lowering workability. PA1 (b) Positional deviations of the PTC thermistor element will possibly occur when unloading the guide films, which in turn makes it difficult to achieve accurate position determination or alignment of the PTC thermistor element. PA1 (c) During insertion (press fitting) of the PTC thermistor element between the guide films, these guide films must rub the electrode of the PTC thermistor element causing the electrodes to become scarred on the surfaces thereof. PA1 (d) Use of the guide films increases costs.